A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatuscan be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The surface of a substrate or patterning device may not be perfectly flat. This unflatness of a substrate or patterning device may have a substantial influence on the imaging accuracy of a lithographic process. In order to take into account the unflatness of a substrate it has been proposed to measure a height map of the substrate before the lithographic process. In a scanning type lithographic, this height map may be used during the lithographic process to correct for height differences in the surface of the substrate by continuous adaptation of the position of the substrate support.
In this method, which is also referred to as leveling, for each location of the projection slit, i.e. the area in which the (patterned) beam is radiated, with respect to the substrate an optimal height and orientation of the substrate may be realized. However, it is not possible to make any corrections for differences in height within the area of a projection slit. Since for each projection slit only one position, i.e. height and orientation, of the substrate with respect to the projection beam may be used.
It has been proposed to use a patterning device bending device to influence the curvature of a patterning device. Such patterning device bending device may be used for optimization of the patterned projection beam within the projection slit area. With such patterning device bending device, the curvature of the patterning device may be adapted to the curvature of the substrate, in particular the area of the substrate on which a patterned radiation beam is projected. A patterning device bending device is disclosed in US 2008-0013068, the contents of which is herein in its entirety incorporated by reference.